This day, Maundy Thursday (also "Holy Thursday" or "Shire Thursday"1) commemorates Christ's Last Supper and the initiation of the Eucharist. Its name of "Maundy" comes from the Latin word mandatum, meaning "command." This stems from Christ's words in John 13:34, "A new commandment I give unto you." It is the first of the three days known as the "Triduum," and after the Vigil tonight, and until the Vigil of Easter, a more profoundly somber attitude prevails (most especially during the hours between Noon and 3:00 PM on Good Friday). Raucous amusements should be set aside...
As to customs, many families have a practice of visiting the tabernacles of three or seven nearby churches after the Mass on this day as a sort of "mini-pilgrimage" (any nearby Catholic churches will do). Some families visit the churches directly after the evening Mass; others go home and wake up in the middle of the night to make the visits (though since churches are rarely open all night these days, this would be hard to do). The spirit of the visits to the churches is keeping vigil in the Garden of Gethsemani while Jesus prayed before His arrest. Matthew 26:36 "Then Jesus came with them into a country place which is called Gethsemani; and he said to his disciples: Sit you here, till I go yonder and pray."
In Germany, Maundy Thursday is known as "Green Thursday" (Grundonnerstag), and the traditional foods are green vegetables and green salad, especially a spinach salad. In Latin countries, Jordan almonds ("confetti") are eaten today and also throughout Eastertide.
Back when Kings and Queens of England were Catholic, they, too, would wash the feet of 12 subjects, seeing the footwashing rite also as an example of service and humility. They would also give money to the poor on this day, a practice is said to have begun with St. Augustine of Canterbury in A.D. 597, and performed by Kings since Edward II. Now the footwashing isn't done (it was given up in the 18th c.), but a special coin called "Maundy Money" is minted and given to the selected elderly of a representative town.
On this day, one may gain a plenary indulgence, under the usual conditions, by reciting the Tantum Ergo (Down in Adoration Falling).
The name "Shire Thursday" is explained in "Festival" printed by Wynkyn de Worde in 1511: "Yf a man aske why Shere Thursday is called so, ye may saye that in Holy Churche it is called (Cena Domini) our Lordes Souper daye; for that day he souped with this Discyples openly; and after souper he gave them his flesshe and his blode to ete and drynke. It is also in Englysshe called Sher Thursdaye, for in olde faders dayes the people wold that daye sher there heedes, and clyppe theyr berdes, and poll theyr heedes, and so make them honest ayenst Ester Day."